The final stage in the production of yellowcake (U.sub.3 O.sub.8) involves the removal of water from a yellowcake slurry. Two methods of drying have been used to accomplish this removal. One method consists of a direct-fired system wherein the yellowcake slurry is fed into a dryer and subsequently comes in contact with the combustion products of the gas. The high temperature and mass flow of the combustion gases vaporize the water and remove it from the slurry. The steam and combustion gases which result from this process are exhausted to the atmosphere through air pollution control equipment. With this method, some of the yellowcake dust entrained in the steam and combustion gases is recovered but not the heat. The other drying method utilized is an indirect-fired system. There, hot oil (600.degree. F.) is circulated through the walls of a vessel containing the slurry. The heat is transferred from the vessel into the slurry vaporizing the water contained therein. This approach produces a cleaner off-gas consisting of 98 percent steam and 2 percent noncondensables, e.g., Cl, SO.sub.x, and entrained yellowcake dust. Again the gases are cleaned with air pollution control equipment. Both of these drying methods typically employ either a venturi type scrubber or a baghouse for cleaning the off-gases produced in the dryer.
Although scrubbers or baghouses are capable of recovering 98 percent of the yellowcake entrained in the off-gases, they are subject to the following problems and disadvantages. Both generate emissions which necessitate obtaining state and federal permits. Although operating at 98 percent efficiency, either type still loses considerable amounts of yellowcake--in the neighborhood of ten thousand pounds annually for a process producing yellowcake at a rate of three million pounds annually. Because of problems such as line plug-ups or blinding filter bags, these systems require constant maintenance resulting in costly production process downtime. The high negative pressure (12 to 35 inches W.G.) fans used with these systems produce more air infiltration into the dryer than is desired, thereby increasing dryer inefficiencies. In addition, control of the dryer draft produced by these fans is difficult to achieve in off-gases laden with water and dust. The controls and dampers become caked up and are quickly rendered inoperable, resulting in drawing too much air through the dryer which causes loss of additional yellowcake product by entrainment. Baghouses and scrubbers do not have any provisions or effect any conditions which make heat recovery convenient because of the contamination of their effluent with yellowcake dust. Scrubbers also add water to the process during recovery of yellowcake dust particles. This radioactive water must be removed elsewhere in the process at additional expense. Because high energy scrubbers (venturi type) are required to achieve 98 percent efficiency, more horsepower is necessary for their operation, thus increasing capital and operating costs.
In venturi-type scrubber systems used for the recovery of yellowcake particles, the off-gases produced in the dryer are drawn into the scrubber for cleaning. The spent yellowcake-containing spray from the scrubber is collected in a sump and is recirculated to the scrubber. A portion of the sump water recirculated is bled off into a thickener to permit recovery of the yellowcake particles contained therein. Because some water is lost to the atmosphere through evaporation, makeup water is added to the sump in order to replace this lost water and to prevent the concentration of yellowcake particles in the sump from exceeding 3 percent solids. Higher concentrations of yellowcake particles decrease the efficiency of the scrubber. In a typical system, the volume of off-gases produced in the dryer is usually too low to be effectively cleaned in the venturi scrubber. Accordingly, ambient air is combined with the off-gases to increase the volume of air and off-gases to a level suitable for cleaning. For example, in a yellowcake dryer having a slurry feed rate of about 1480 lbs./hr., approximately 740 lbs./hr. of off-gases and steam are produced. These gases along with air, which infiltrates the dryer at a rate of 225 SCFM, exit the dryer at a total flow rate of about 551 SCFM at 392.degree. F. Approximately 509 SCFM of ambient air is added to this amount in order to have a flow rate of 1060 SCFM through the scrubber--the total volumetric rate required for effective cleaning of the off-gases.
For the scrubber process as described, the amount of losses are about one million Btu/h of heat energy and 21/2 lbs./hr. of yellowcake when the yellowcake process is producing at a capacity of about 740 lbs./hr. of dried yellowcake. In addition, approximately 31 horsepower is consumed by this system, whereas in one embodiment of the present invention only 15 horsepower is consumed.
Although the above process deals with the recovery of yellowcake, this process does not address itself to the trifold recovery of heat, process fluid, and process product which would otherwise be lost.